Anophthalmia refers, strictly speaking, to absence of the globe and ocular tissue from the orbit. However, in most cases of anophthalmia, CT scans show some remnants of the globe, indicating they represent severe microphthalmia (small eyes).
Anophthalmia/microphthalmia may involve one or both eyes. It may occur in isolation with no associated malformations or it may be part of a multiple congenital malformation syndrome as, for example, the congenital rubella syndrome, the triploidy syndrome (due to the presence of 69 chromosomes), the trisomy 13 syndrome (Patau syndrome, due to an extra chromosome 13), and the Wolf-Hirschorn syndrome (which is due to deletion of part of the short arm of chromosome 4).
From the Greek an-, absence + ophthalmos, eye + absence of the eye.